1. Field of the Invention
The invention relates to a multi-point communication system for performing communication by connecting communication stations at multiple points and to a communication terminal station apparatus constructing such a system.
2. Related Background Art
In recent years, the development of the image compression coding technique and the spread of the digital communication line have been remarkable. The recommendations of the service regulations and protocol regulations for AV (Audio Visual) services in a TV conference system or the like, regulations for multimedia multiplexing frame construction, and the like are completely provided. Various terminal stations as well as TV telephone apparatus, TV conference systems, etc. have been proposed.
Among them, a multi-point TV conference system for performing a conference by connecting terminal stations at three or more points is highlighted.
In the multi-point TV conference system, generally, a multi-point TV conference is realized by mutually connecting the TV conference terminal stations by using an inter multi-point communication control apparatus (hereinafter, simply abbreviated to an MCU).
There has also been proposed a system for realizing a multi-point TV conference system by connecting TV conference terminal stations to a communication line in a ring or chain shape and by relaying predetermined data at each station. Specifically speaking, a data region is allocated to each station and is circulated, thereby updating the data inserted in a local station at every station.
The conventional multi-point TV conference system as mentioned above, however, has the following problems.
First, in the former system, since the multi-point TV conference system is realized by connecting all of the TV conference terminal stations to the MCU and by monitoring and controlling all of the TV conference terminal stations and the portions among them by the MCU, there are the following problems.
(1) The conference cannot be realized in the case where the MCU is in an inoperable state, the case where the MCU is used by another multi-point TV conference, or the like.
(2) Since a plurality of communication control circuits and the like for connecting a plurality of stations are needed, the MCU main body is very expensive.
(3) There is a limitation in the number of participants of the conference depending on a terminal station (line) accommodating ability of the MCU.
On the other hand, in the latter system, namely, in the system such that the TV conference terminal stations are connected to the communication line in a ring or bus shape and the data region is allocated to each station and circulated data is relayed and the data region of the local station is updated, although there is an advantage such that the MCU is unnecessary, since all of the data is circulated, the data stored in the local station is also received at each station. It will be obviously understood that since the reception of the data inserted by the local station is a substantially meaningless operation, the data which essentially has no meaning always occupies a part of the line.
In a digital communication network represented by the ISDN (Integrated Services Digital Network) or the like, although bidirectional transmission can be performed between the stations, in the conventional system, two loops in the opposite directions are not always efficiently used. Particularly in the transmission of moving image information, consequently, since the information amount is extremely large, there occur problems such that not only is the line not effectively used but also there is a case where another useful information cannot be transmitted.
The above problems generally occur without limitation to the moving image or the image communication.